100% recyclable composites – Researchers of the LZN develop new lightweight materials

Partners of the University of Freiburg Freiburg, Fraunhofer-Institute for Mechanics of Materials (IWM), and the industry have developed a pure polyethylene composite. After multiple experiments, they were able to identify the perfect combination of various polyethylene molecules and to analyze this combination using injection molding simulations. They thereby discovered a plethora of possible applications, which are made possible by the unique characteristics of the so-called “polymer blend”. What makes them unique? The currently used composites are generally made of synthetic materials in which fibers, for example carbon fibers, are embedded. These composites thereby exhibit great characteristics, such as a high stability. However, they are generally non-recyclable or require extremely large amounts of energy to be recycled. A pure composite, in which the fibers are made of the same material as the matrix, would solve this problem.

Lightweight construction research is becoming increasingly important in the scientific community. Researchers worldwide are inspecting and creating novel composites, combinations and materials, trying to optimize them for various applications. Within the group of lightweight construction materials, one can find different mixes of synthetic materials such as specific forms of high performance polymers. These can include polymer fibers with outstanding mechanical characteristics. A well-known example of such a high performance polymer is PPTA (poly(p-phenylenterephtalamid)) which is the main component of Kevlar®. The polymer chains in such materials are oriented in the direction of the mechanical burden, which leads to the desired mechanical characteristics such as stiffness, robustness, etc. These high performance fibers can be manipulated into fabrics, laminates, and fiber bundles. This is optimal for lightweight construction as these fibers and the matrix around them are much lighter than metals with the same mechanical properties. However, the recycling of such fibers is generally problematic. Polyolefin polymers are comparatively easy to recycle: they are also excellent materials for lightweight construction as they offer great versatility. Plus, they are produced in an efficient polymerization process which requires little energy and resources. This process does not result in any environmentally damaging secondary products nor solvents. The so-called “carbon footprint” of this production is thereby small and the low monetary, ecological and energy costs of it contribute to the attractiveness of this process. However, in order to compete with metals, polyolefin polymers also require filler material or other fibers. With this combination of different materials, the problem of recyclability arises once more.

A genius solution to this problematic was proposed by Prof. Dr. Rolf Mülhaupt, director of the Institute for Macromolecular Chemistry at the University of Freiburg and world-renowned chemist. He wanted to develop and characterize an “all polyethylene”, 100% recyclable composite material, in an ecologically and energetically efficient process. Mülhaupt joined partners from Fraunhofer IWM and the company Lyondellbasell for the pilot project “susCOMP: Molecular Composites for Sustainable Light Weight Construction”. The project is part of the Sustainability Center Freiburg (LZN). Its goal was to examine various polyethylene reactor blends in order to determine, which resulting mix would be best suited for sustainable lightweight construction.

“Mülhaupt had the idea to create a mix out of PE (polyethylene), UHMWPE (ultra-high-molecular-weight polyethylene) and PE-Wax and use that as a composite material,” acknowledges Dr. Raimund Jaeger from Fraunhofer IWM. “And in order to get the right mixture, certain skills are definitely required.” The mixture of different polymers is described henceforth as the “blend”, which, as compared to the conventional polymers, is characterized by completely different properties i.e. exactly what the researchers were hoping. The resulting mixture is suitable for injection molding and forms the basis of various structures with vertically oriented fibers, such as the Shish-Kebab structure depicted in Figure 01.

Within susCOMP, the researchers wanted to examine new possibilities to improve the characteristics of blends made of only one polymer. First, they wanted to successfully create the blend of PE, UHMWPE, PE-Wax. Secondly, they wanted to examine, what properties a mixture of HDPE (High Density Polyethylene) and a binary reactor blend would display. Next, they wanted to produce a reactor blend with tribology additives. Fourthly, they aimed to examine the mechanical properties of the reactor blends, particularly if they changed during collision stresses. In order to be an accepted substitute for classical composites, the pure composites would have to manage the same stresses equally well if not better. To this question, the researchers added a further goal of developing suitable material models. As their fifth and final goal, the susCOMP team wanted to analyze the tribological properties of the reactor blend, especially in terms of friction and abrasion. As an overarching guideline, the team needed to keep the structure-properties relationship in mind. These questions were to be answered experimentally and with numerical simulations.

The researchers hoped that the PE-wax would act like a lubricant. In order to reach their first goal, the team tried various mixes of the three molecules to finitely determine, which mixture would present the best characteristics. The initial results were as Dr. Jaeger describes “not surprising. The more UHMWPE was present in the mixture, the better its mechanical properties.” This could be determined among others, during the observation of the abrasion rate which significantly improved when more UHMWPE was present. These results validated those from previous studies found in the literature. Previously undiscovered however, was that one of the blends, which went in the direction of a polyamide, a specific polymer structure, had significantly better properties than pure HDPE.

Dr. Jaeger is very pleased with these initial results. “The application of these pure composites is very promising, we could build all sorts of structures from it, such as ‘molecular superstructures.’ However, there are certain limitations that we still need to address.” One of these is the melting point of the blend. The crystalline structures can only maintain their mechanical properties up to 80°C which means, they are useless at higher temperatures. This makes their application in safety-critical areas impossible, as the temperature cannot be guaranteed to remain under the melting point at all times.

A positive attribute of the developed blend, is that an “up-cycling” procedure can be carried out on the product. Up-cycling is the process in which a product can be re-used in a completely different utilization, sparing resources. For example, the filler materials used in asphalt production are re-used in a further step as materials in the automotive industry. Since all parts of the structures are made of the new all-polyethylene blend, they can be converted back to their basic foundation and then re-used in whatever process that may require the same materials. In principle, the structures are completely reusable since at the end of the conversion, “virgin material” is left over, as Dr. Jaeger explains.

Within the last three years, Mülhaupt, Jaeger and their colleagues were able to test approximately 30 different composites, but there is still much to examine and learn. Thus, the researchers are planning to initiate a follow-up project. All the more, as industry shows great interest in their results, already.